In a capacitive speaker that utilizes a piezoelectric transducer mechanism or electrostatic transducer mechanism, e.g., a piezoelectric speaker, a condenser speaker or a flat-panel speaker that employs a piezoelectric actuator, it is known that conductance and susceptance increase as the frequency of the input signal rises. FIG. 11 is a graph illustrating an example of the frequency characteristics of conductance and susceptance of a capacitive speaker. A curve 1 in FIG. 11 indicates the conductance characteristic of the capacitive speaker, and a curve 2 in FIG. 11 indicates the susceptance characteristic of the capacitive speaker.
In an audio circuit for driving a capacitive speaker, therefore, it is necessary to supply a current that is large in comparison with signal components having a high frequency (referred to as “high-frequency components” below) within the frequency band of a signal (referred to as an “audio signal” below) corresponding to voice or music to be reproduced. For this reason, an increase in power consumption is a problem in an audio circuit for driving a capacitive speaker, in a capacitive speaker system having this audio circuit and capacitive speaker, and in an electronic apparatus having this capacitive speaker system. Further, driving a capacitive speaker based upon an audio signal that contains many high-frequency components also invites a decline in the efficiency of the drive amplifier and the destruction thereof.
Accordingly, in a conventional audio circuit, input of high-frequency components to a capacitive speaker is attenuated by providing a drive amplifier that drives the capacitive speaker or the input side of the drive amplifier with a low-pass filter or band-pass filter formed using passive elements such as a capacitor, resistor and coil and active elements such as transistors.
FIG. 12 is a block diagram illustrating the configuration of a conventional audio circuit.
As illustrated in FIG. 12, the conventional audio circuit comprises a signal generating source 3 for reading an audio signal from a recording medium or the like; a DSP (Digital Signal Processor) 4 for applying processing to the audio signal that has been output from the signal generating source 3, the DSP including a demodulating circuit and a decoding circuit as well as various sound-effect processing circuits that operate as an equalizer and as limiter, etc., for compressing the audio signal to within a fixed level; a D/A converter 5 for converting the output signal of the DSP 4, which is a digital signal, to an analog signal; a first filter 6 for attenuating high-frequency components of the audio signal that has been output from the D/A converter 5; a drive amplifier 7, which includes a preamplifier, etc., for driving a capacitive speaker based upon the output signal of the first filter 6; a second filter 8 for attenuating high-frequency components or blocking high-frequency noise in the signal that has been output from the drive amplifier 7; and a capacitive speaker 9 for reproducing voice or music, etc., based upon the signal supplied from the second filter 8. A low-pass filter or band-pass filter is used as the first filter 6 and second filter 8.
It should be noted that circuits for driving a capacitive speaker (piezoelectric speaker) are described also in Patent Document 1 and Patent Document 2, by way of example.
[Patent Document 1]
Japanese Patent Kokai Publication No. JP-A-59-146296
[Patent Document 2]
Japanese Patent Kokai Publication No. JP-P2002-369290A